A geospatial approach to targeting constructed wetlands for nitrate removal in agricultural watersheds

Constructed wetlands are an effective yet costly strategy for reducing nitrate loading from agricultural subsurface drainage in the Midwestern United States. Targeting wetland placement to sites that intercept high nitrate loads and where topography permits optimal wetland sizing can maximize wetland nitrate removal efficiency while maintaining productive agriculture. The goals of this work were first to determine targeted locations for constructed wetlands to remove nitrate by applying a geospatial approach within an Indiana study region, and second to estimate the nitrate removal efficiency of these proposed wetlands. Criteria were developed from previously published wetland siting criteria associated with the Iowa CREP program and adapted to Indiana-specific conditions. Final criteria included location on cropped land and not within open waterways, size of upland contributing area, presence of tile drainage, and wetland-like topography. Within the study region, 18 locations were found to meet the targeting criteria for strategic wetland placement, requiring land conversion of 0.08% of the study region and intercepting 2.7% of flow from tile-drained lands. Estimated wetland nitrate removal efficiency was on average 37%, including land conversion of wetland and surrounding buffers, resulting in the removal of approximately 1.0% of all nitrate currently exported from tile-drained land in the study area. Wetland placement was most influenced by criteria relating to contributing area, exclusion of streams where wetlands could be potentially located, and topography. Adjusting criteria could allow for a larger set of constructed wetlands and achieve greater overall nitrate reductions at the watershed scale, but this set would likely include less efficient wetlands.